if you have a bit of left overs to restart a new batch honestly not long 20mins-45mins
but if new with fresh distilled water it takes longer.
Now that all depends on voltage, volume, amperage, distance between conductors but i routinely run too long myself but not by choice, I just let things settle and decant off any sediment when im done and I only usually make 4oz at time “spray bottles worth”
What to look for is once you start to see gray/black particles forming and starting to drop with usally a silver sheen starting to form on the surface your usually already past done cause by that point the particles are too big. I forget the color change order but it goes white, yellow, red, then grey/black when looking at it from the side with a light.
Usually its right at the end of the red stage your typically done, the color shift happens because the particle size is increasing so the reflection of light changes